In recent years, capacitive electromechanical transducers manufactured through a micromachining process have been researched actively (see Japanese Patent Application Laid-Open No. 2006-319712). A normal capacitive electromechanical transducer has a lower electrode, a membrane (diaphragm), which is supported to keep a given distance from the lower electrode, and an upper electrode, which is disposed on a surface of the membrane. This is known as, for example, a capacitive micromachined ultrasonic transducer (CMUT).
The above-mentioned capacitive micromachined ultrasonic transducer uses a light-weight membrane to send and receive ultrasonic waves, and it is easy to obtain a capacitive micromachined ultrasonic transducer that has excellent broadband characteristics in liquids as well as in the air. The CMUT enables more accurate diagnosis than conventional medical diagnosis, and is therefore beginning to attract attention as a promising technology.
The operation principle of the capacitive micromachined ultrasonic transducer is described. To transmit ultrasonic waves, a DC voltage overlapped with a minute AC voltage is applied between the lower electrode and the upper electrode. This causes the membrane to vibrate, thereby generating ultrasonic waves. When ultrasonic waves are received, the membrane is deformed by the ultrasonic waves, and the deformation causes a capacity change between the lower electrode and the upper electrode, from which signals are detected.
The theoretical sensitivity of the capacitive electromechanical transducer is in inverse proportion to the square of the distance between its electrodes (“gap”) (see IEEE Ultrasonic Symposium, 1997, p. 1609-1618). It is said that a gap of 100 nm or less is necessary to manufacture a highly sensitive transducer, and the “gap” in the CMUT has recently been made as narrow as 2 μm to 100 nm or less.
A commonly employed method of forming the “gap” in the capacitive electromechanical transducer includes providing a sacrificial layer that is as thick as a desired gap between the electrodes, forming the membrane on top of the sacrificial layer, and then removing the sacrificial layer. This and similar technologies are disclosed in U.S. Pat. No. 6,426,582, Japanese Patent Application Laid-Open No. 2005-027186, Japanese Patent Application Laid-Open No. H06-216111, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, No. 12, December 2005, p. 2242-2258.